Atomic power facilities include parts which move relative to each other and which require lubrication, such as hydraulic actuators, hydraulic shock absorbers, vibration insulators, and other parts which are exposed to radioactive rays. Working oil useful in lubricating such parts of atomic power facilities are generally required to possess high resistance to radioactive rays. In consequence of the recent growth of the nuclear power industry, the resistance of lubricants to radioactive rays in attracting serious attention. The issue has placed an impetus on studies directed to providing new oils possessing high resistance to radioactive rays sufficient for them to be used as such working oil.
As substances having high resistance to radioactive rays, therehave heretofore been discovered polycyclic aromatic condensation compounds such as naphthalene, anthracene and pyrene; polyphenyls such as diphenyl and terphenyl; and polyphenyl ether. However, all these compounds are solid at normal room temperature. In order for these substances to be used as working oil, therefore, it is necessary that they should be dissolved in solvents or melted under heating so as to assume a liquid state. In dissolving the substances by the use of solvents, since no solvents have so far been discovered which have resistance to radioactive rays comparable with that offered by the substances, use of the solvents inevitably results in degradation of the resistance of the substances to radioactive rays. In converting the substances into a molten state under heating, it becomes necessary to incorporate in the lubrication system, a special device for heating. Because of these disadvantages, substances which are solid at room temperature are not feasible for use as working oil. Separately, working oils of petroleum origin (such as, for example, APL-710 produced by Shell Petroleum Co.) or phenyl silicone oils (such as, for example, F-4 produced by Shin-etsu Chemical), which are liquid at room temperature, have heretofore been utilized also as working oil for the aforementioned purpose.
However, the lubricants of petroleum origin mentioned above tend to decompose with simultaneous evolution of gas while in use. This phenomenon of gas evolution is quite troublesome in cases where the oil is used in a closed system especially in a high-speed propagating type or nuclear reactors in which an anti-shock and anti-vibration apparatus containing such an oil is provided on a pipe-line for melted sodium of the primary cooling system of the nuclear reactor.
Recently it was found that sodium in the pipe-line becomes to radioactive .sup.24 Na by the irradiation of neutrons generated in the reactor, and the oill in the anti-earthquake apparatus is deteriorated by gamma ray from .sup.24 Na to generate a large amount of a gas mainly composed of hydrogen. Although the deterioration of the oil, which changes the physical properties of the oil to the degree where the deteriorated oil can not exhibit the original performance of the initial oil is a large problem, the more important result is the afore-mentioned large volume of the gas generated from the deterioration, because the anti-earthquake apparatus containing such an oil is a closed vessel. Accordingly, the accumulated gas in the closed vessel of the anti-earthquake apparatus strongly hinders the performance of the apparatus. That is why the amount of the gas from the irradiated oil is widely tested in the selection of the oil to be used in such an anti-earthquake apparatus. This test is carried out by subjecting the candidate oil to the exposure of gamma ray from .sup.60 Co in an atmospheric condition and also under highly reduced 5.times.10.sup.-4 mmHg pressure at room temperature, the amount of exposure is 10.sup.8 R, 10.sup.9 R and 3.times.10.sup.9 R, in order of screening the candidate oils by a dose rate of 1.6.times.10.sup.6 R/hour (for the determination of the degree of deterioration of the oil) and 0.96.times.10.sup.6 R/hour for the determination of the amount of generated gas. The volume of the gas is calculated into more, then into the number of molecules of the generated gas. As is shown in Tables in the original specification, G value represents the number of molecules of the generated gas when the oil absorbed the radiation energy of 100 eV. In this case it is impossible to obtain the electron density of the oil, therefore, it is presumed that the electron density of the oil is nearly equal to that of water (H.sub.2 O). Accordingly, the energy absorbed by 1 g of the oil when it is subjected to the radiation of R roentgen is nearly 6.08.times.10.sup.13 (unit being eV/R).
The G value is given by the following formula: ##EQU1##
In short, the G value is an index of the total volume (amount) of gas generated from a candidate oil during exposure to total radiation of R roentgen units. The larger the G value the less stable the candidate oil against radiation.
While the phenyl silicone oils tend to gel while serving as a working oil. Thus these oils also fail to serve as a satisfactory working oil resistant to radioactive rays.
Under the circumstances described above, there has been felt the strong necessity for developing an oil which has a high resistance to radioactive rays comparable with that of polyphenyl and its equivalents and which is liquid at room temperature.